Coating forming method and coating forming material, and abrasive coating forming sheet

ABSTRACT

A brazing filler metal sheet is prepared. The brazing filler metal sheet includes a brazing filler metal layer, a sticking material layer, and a released paper. The brazing filler metal layer includes a brazing filler metal. A coating material layer is laminated on the brazing filler metal layer. The coating material layer includes a mixture of coating material particles and a binder. As the coating material particles, MCrAlY particles and abrasive particles are used. The coating material layer is then dried, and the brazing filler metal sheet is cut (step S 4 ), and adhered to a rotor blade. The rotor blade is heated, to melt the brazing filler metal. The brazing filler metal diffuses due to the heat treatment holding process. A solidified layer is then formed by cooling. This solidified layer is subjected to blasting to allow the cubic boron nitride particles to protrude.

TECHNICAL FIELD

This invention relates to a method of forming an abrasive coating, anoxidation-resistant coating or the like on, for example, a rotor blade,a stator blade, or a shroud in a combustion engine (gas turbine, jetengine, and the like) or a steam turbine, a coating formation material,an abrasive coating formation sheet, and a rotor blade in a gas turbineon which an abrasive coating or the like is formed by this coatingformation method, and a gas turbine using this rotor blade.

BACKGROUND ART

In a gas turbine, a predetermined clearance is provided between a tip ofthe rotor blade and the shroud that faces the tip of the rotor blade, sothat the tip of the rotor blade and the shroud do not come in contactwith each other during operation. If this clearance is too large,combustion gas leaks from the pressure surface side to the suctionsurface side of the rotor blade, thereby the combustion gas that can beused for driving the turbine decreases. As a result, the operationefficiency of the gas turbine decreases. Therefore, the clearance is setas small as possible, for suppressing the leak of the combustion gas asmuch as possible, to improve the performance of the gas turbine.

However, if the clearance is too small, in the initial stage of startupof the gas turbine, the tip of the rotor blade and the shroud may slidewith each other, resulting from thermal expansion of the rotor blade,eccentricity of a turbine rotor, vibrations occurring in the whole gasturbine, or the like (a so-called initial sliding). When the gas turbineis operated for long time, the shroud exposed to the high-temperaturegas gradually causes a thermal deformation, thereby the tip of the rotorblade and the shroud may slide with each other (a so-called secondarysliding).

In general, the shroud comprises a coating as a thermal insulation orantioxidation on the internal peripheral face thereof. For example, athermal barrier coating (TBC) may be provided for thermal insulation, oran antioxidant coating consisting of McrAlY may be provided, where M isone or more of iron, nickel, and cobalt. These coatings often have highhardness, and hence, if the tip of the rotor blade and the internalperipheral face of the shroud slide with each other, the rotor blade maybe largely damaged.

Japanese Patent Application Laid-Open Nos. 4-218698 and 9-504340, andU.S. Pat. No. 5,702,574 disclose a rotor blade having an abrasivecoating, in which abrasive particles are dispersed in a matrix ofMcrAlY, which is an antioxidant material. In this rotor blade, forexample, cubic boron nitride (CBN) or the like is used as the abrasiveparticles. The cubic boron nitride is a material having high hardness,and hence, if the rotor blade and the internal peripheral face of theshroud slide with each other, the abrasive particles comprising thiscubic boron nitride polish the internal peripheral face of the shroud.As a result, appropriate clearance can be maintained between the rotorblade and the shroud.

This abrasive coating may be formed as follows. That is, abrasiveparticles are temporarily fixed to the rotor blade body, and a matrix isformed around the abrasive particles by electrodeposition. In otherwords, the matrix is formed by the growth of a deposit. Since the growthof the deposit requires time, this forming method has poor efficiency.Further, the formation of the matrix by the electrodeposition isgenerally expensive. Further, electrodeposition needs large-scaleequipment, and it is difficult to newly build the electrodepositionequipment from a standpoint of environmental protection.

Japanese Patent Application Laid-Open No. 10-30403 discloses an abrasivecoating formation method in which a matrix is formed by a thermalspraying method. The thermal spraying method is a method of allowing ametal layer to grow by injecting a molten metal, and has a feature inthat it is highly efficient as compared with the electrodepositionmethod. In the thermal spraying method, however, when abrasive particlesare temporarily fixed to the rotor blade body, the electrodepositionmethod is used. Therefore, it has the problems described above, and itis also difficult to accurately control the thickness of the matrix, andlarge-scale thermal spraying equipment is required. When abrasiveparticles such as cubic boron nitride are dispersed in the metal matrixby the thermal spraying method, since the abrasive particles are buriedin the molten metal, it is necessary to remove the molten metal untilthe abrasive particles are exposed. However, it is difficult to exposethe abrasive particles, and hence, it becomes difficult for the abrasiveparticles to polish the internal peripheral face of the shroud. Further,the metal matrix may be welded on the internal peripheral face of theshroud, to damage the rotor blade.

An antioxidant coating of the TBC or MCrAlY may be formed on theinternal peripheral face of the shroud. These coatings are generallyformed by the thermal spraying method, such as an atmospheric plasmaspray (APS) method, a high velocity oxygen fuel (HVOF) method, a lowpressure plasma spray (LPPS) method, or a detonation gun (D-GUN) method.

It is an object of the present invention to provide a coating formationmethod, a coating formation material, an abrasive coating formationsheet, a rotor blade in a gas turbine, on which an abrasive coating orthe like is formed by the coating formation method, and a gas turbineusing this rotor blade.

DISCLOSURE OF THE INVENTION

In order to achieve these objects, the coating formation methodaccording to the present invention includes the following steps (1) to(3).

-   (1) a lamination step of laminating a brazing filler metal layer    composed mainly of a brazing filler metal and a coating material    layer composed mainly of a coating material, on the surface or the    back of an object to be coated;-   (2) a melting step of heating the laminated brazing filler metal    layer and coating material layer to diffuse the coating material and    the brazing filler metal, while allowing the brazing filler metal    component to melt and infiltrate in the coating material; and-   (3) a fixing step of solidifying the molten brazing filler metal to    fix it on the object to be coated.

In the coating formation method according to the present invention, acoating is formed by a so-called brazing. This method is cheap ascompared with the plating or thermal spraying method, and does notrequire large-scale equipment, and hence there is little limitation onthe application site.

In this case, it is desired that a coating parameter between the brazingfiller metal and the coating material laminated at the lamination stepbe from 30:70 to 70:30 inclusive, as in the coating formation methodaccording to the present invention. The brazing filler metal is reliablymelted in the coating material at the melting step, by selecting thevolume ratio in this manner.

As in the coating formation method according to the present invention,it is preferred that the brazing filler metal contains boron. Borondiffuses in the coating material at the melting step, to allow thesolidifying point of the coating material to fall. Therefore, even whenthe coating material is heated at a relatively low temperature, thecoating material melts, and once it melts, boron decreases to raise themelting point. As a result, a problem such as remelting hardly occurs inthe actual operation.

As in the coating formation method according to the next invention, thebrazing filler metal is preferably selected from materials having amelting point lower than the heat treatment temperature of the object tobe coated. As a result, the melting step can be executed at the sametime with the heat treatment of the object to be coated.

It is also preferred that the coating material layer to be used is onein which coating material particles diffuse in a binder, as in thecoating formation method according to the next invention. Lamination ofthe coating material becomes easy by the binder. Since the bindervolatilizes substantially completely at the melting step, it issuppressed that the quality of the coating deteriorates due to thebinder remaining in the coating. If a volatile binder is used, itvolatilizes easily at the melting step. Hence, the quality of thecoating can be further improved, by reducing the quantity of the binderremaining in the coating. As the binder, one that volatilizes at a lowtemperature is preferable, and it is also desired to select the binderhaving a certain degree of strength (rigidity) of the coating materialafter the binder has dried.

As in the coating formation method according to the next invention, itis desired that a mass ratio between the binder and the coating materialparticles be from 15:85 to 2:1 inclusive. As a result, formation of thecoating material layer becomes easy, and liquid dripping of the brazingfiller metal at the melting step can be suppressed.

One example of a preferable coating material layer includes onecomprising, as main component, MCrAlY particles and cubic boron nitrideparticles. An abrasive coating can be obtained by this coating materiallayer. In this abrasive coating, cubic boron nitride serves as abrasiveparticles, and MCrAlY becomes a matrix to fix the abrasive particles.The MCrAlY matrix also suppresses oxidation of the abrasive particles orthe rotor blade material.

As in the coating formation method according to the next invention, itis desired that the volume ratio between the MCrAlY particles and thecubic boron nitride particles is from 1:2 to 2:1 inclusive, from astandpoint of consistence of improvement in the polishing ability andreliable fixation of the abrasive particles.

As in the coating formation method according to the next invention, ifthe abrasive coating is formed at the tip of the rotor blade of a gasturbine, the abrasive coating polishes the internal peripheral face ofthe opposite shroud, and hence a damage of the rotor blade by adhesioncan be prevented.

In this coating formation method, it is desired to include an exposurestep of removing a part of MCrAlY from the surface of the fixed coatingmaterial layer to expose the cubic boron nitride particles, as in thecoating formation method according to the next invention.

The preferable exposure method is blasting, as in the coating formationmethod according to the next invention. As in the coating formationmethod according to the next invention, it is desired that in theblasting, an abrasive harder than the MCrAlY particles but softer thanthe abrasive particles be used. As a result, since MCrAlY can be removedefficiently from the formed abrasive coating, the abrasive particles canbe exposed sufficiently.

In the blasting, as in the coating formation method according to thenext invention, it is desired that the particle size of the abrasive issmaller than that of the abrasive particles and smaller than the spacebetween the abrasive particles. However, if the particle size is madetoo small, the abrasive particles attack the holder of the abrasiveparticles to cause a dropout, and hence precautions should be takenregarding this point. As a result, dropout of the abrasive particles canbe suppressed to a minimum, while sufficiently exposing the abrasiveparticles, and hence sufficient polishing performance can be exhibitedfrom the initial stage.

Other examples of the preferred coating material layer include onecomposed mainly of the MCrAlY particles, as in the coating formationmethod according to the next invention. A coating having an oxidationresistance and an intergranular corrosion resistance obtained by thiscoating material layer can be preferably used in various members of agas turbine where high-temperature gas circulates, more specifically, ina rotor blade, a stator blade, and a shroud, as in the coating formationmethod according to the next invention.

A coating formation coating material according to the next inventioncontains abrasive particles such as cubic boron nitride, Al2O3, SiC, orthe like, a metal material having at least an oxidation resistance, anda binder. Since this coating formation coating material containsabrasive particles, a metal material, and a binder, the brazing fillermetal is absorbed in the gap produced by volatilization of the binder,in the heat treatment at the time of coating formation. Thereby,dripping of the brazing filler metal to the surroundings can beconsiderably reduced, and hence the quality (uniformity of the coatingthickness) after forming the coating on the object to be coated can beimproved. As a result, since adjustment of the coating thickness afterforming a coating can be kept to a minimum, the time and energy forcoating formation can be reduced.

The object to be coated of the present invention includes a rotor bladeand a shroud of a gas turbine. Since these objects to be coated are usedin an atmosphere where high-temperature combustion gas is injected, thelife thereof becomes short because of the reduced thickness due tooxidation. However, since the metal material contained in the coatingformation coating material according to the present invention has anoxidation resistance, oxidation hardly occurs even in such anatmosphere. Therefore, the abrasive particles can be reliably held todemonstrate stable polishing performance, even in long-term use thereof.Further, it has an effect of reducing reduction of thickness of the basemetal due to oxidation, and hence more stable operation of the gasturbine can be realized.

In a coating formation coating material according to the next invention,in the coating formation coating material, a ratio between the mass ofthe binder and the mass of the abrasive particles and the metal materialis from 15:85 to 2:1 inclusive. As a result, the coating material layercan be formed easily, and dripping of the brazing filler metal at themelting step can be suppressed.

In a coating formation coating material according to the next invention,in the coating formation coating material, the metal material is MCrAlY.Since MCrAlY having an oxidation resistance is used as the metalmaterial for forming a coating, even when a coating is formed on therotor blade of a gas turbine used in a high-temperature oxidativeatmosphere, the abrasive particles can be held for long time to maintainthe polishing performance, and to protect the base metal from oxidation.As a result, stable operation of the gas turbine can be realized.

In a coating formation coating material according to the next invention,in the coating formation coating material, the volume ratio between theMCrAlY particles and the abrasive particles is from 1:2 to 2:1inclusive. If the ratio of the cubic boron nitride, Al2O3, or SiC usedas the abrasive particles is large, the content of MCrAlY decreases, andhence not only the oxidation resistance decreases, but also insufficientbrazing filler metal easily occurs at the time of application. Further,holding of the abrasive particles becomes insufficient during brazing,thereby causing a relief of particles. On the other hand, if the ratioof MCrAlY is too large, the polishing ability of the abrasive coatingmay be insufficient. From these points of view, if the mass ratio iswithin the range described above, the occurrence of insufficient brazingfiller metal can be prevented, and the workability can be improved.Further, since the oxidation resistance of the metal layer that holdsthe abrasive particles sufficiently is high, the particles can be stablyheld for long time, and dropout of the abrasive particles can besuppressed, thereby enabling reliable operation of the gas turbine.

In an abrasive coating formation sheet according to the next invention,a brazing filler metal and any one of the coating formation coatingmaterials described above are laminated. In this abrasive coatingformation sheet, since a binder is contained in the coating formationcoating material, the brazing filler metal is sucked in the space wherethe binder volatilizes, in the heat treatment at the time of coatingformation. As a result, liquid dripping at the time of coating formationcan be considerably reduced, and hence the quality after the coating hasbeen formed on the object to be coated can be improved. Since theadjustment after coating formation can be kept to a minimum, the timeand energy for coating formation can be reduced. This abrasive coatingformation sheet is adhered to the object to be coated, and then theabrasive coating can be formed only by heat-treating the object to becoated, and hence the abrasive coating can be formed very easily, ascompared with the plating or thermal spraying method. Further, if ametal material having an oxidation resistance and an intergranularcorrosion resistance is used as the coating formation coating material,even when the abrasive coating is formed on the rotor blade, the shroudand the like in the gas turbine, which are used in a high-temperatureoxidative atmosphere, dropout of the abrasive particles can besuppressed to thereby maintain stable polishing performance. As aresult, stable operation of the gas turbine can be realized.

Since the treatment prior to the heat treatment is completed only byadhering this abrasive coating formation sheet to the object to becoated, the work becomes very easy. Further, since it is a sheet form,it can be appropriately cut according to the shape of the object to becoated. Therefore, it can easily correspond to objects to be coatedhaving various shapes.

In an abrasive coating formation sheet according to the next invention,in the abrasive coating formation sheet, the coating parameter betweenthe brazing filler metal and the coating formation coating material isfrom 30:70 to 70:30 inclusive. By selecting the volume ratio, not onlythe coating formation coating material reliably melts at the meltingstep, but also the formed coating becomes strong.

In an abrasive coating formation sheet according to the next invention,in the abrasive coating formation sheet, boron is contained in thebrazing filler metal. Since boron is contained, at the melting step,this boron diffuses in the coating formation coating material, to allowthe solidifying point of the coating formation coating material to fall.Therefore, even when the coating formation coating material is heated ata relatively low temperature, the coating formation coating materialmelts. After boron diffuses, since the melting point of the coatingformation coating material increases, the heat resistance of the brazingfiller metal increases. As a result, even when the coating formationcoating material is used in a high-temperature gas, such as in the rotorblade and the shroud of the gas turbine, the brazing filler metal can beused without remelting.

In an abrasive coating formation sheet according to the next invention,in the abrasive coating formation sheet, the brazing filler metal isselected from materials having a melting point lower than the heattreatment temperature of the object to be coated. As a result, themelting step is allowed to progress at the same time with the heattreatment of the object to be coated.

In an abrasive coating formation sheet according to the next invention,in the abrasive coating formation sheet, an adhesive layer is formed onthe brazing filler metal. Therefore, so long as the abrasive coatingformation sheet is prepared, the treatment prior to the heat treatmentis completed only by adhering the abrasive coating formation sheet onthe object to be coated, without requiring pasting and waiting fordrying of the paste. As a result, time and energy for coating formationcan be reduced.

In a rotor blade of a gas turbine according to the next invention, acoating is formed at the tip thereof by any one of the coating formationmethods. Therefore, the abrasive coating can be formed very easily, ascompared with the plating or thermal spraying method. As a result, thetime required for coating formation can be considerably reduced, ascompared with the coating formation method described above, and theproduction cost thereof can be reduced.

In a rotor blade of a gas turbine according to the next invention, anyone of the abrasive coating formation sheets is adhered to the tipthereof. Therefore, the abrasive coating can be formed only byperforming the necessary heat treatment on the rotor blade, and hencethe abrasive coating can be formed very easily, as compared with theplating or thermal spraying method. As a result, the time required forcoating formation can be considerably reduced, as compared with thecoating formation method described above, and the production costthereof can be reduced.

A gas turbine according to the next invention comprises: a compressorthat compresses air to produce combustion air; a combustor that allowsthe combustion air produced by the compressor to react with a fuel, togenerate a high-temperature combustion gas; and a turbine having a rotorblade driven by the combustion gas injected from the combustor to therotor blade.

Therefore, so long as the heat treatment equipment is provided, theabrasive coating can be formed easily, and hence the equipment forcoating formation becomes simple, as compared with the plating orthermal spraying method. Hence, even when there is no plating equipmentnear the gas turbine plant, the abrasive coating can be easily formed,if only a heating furnace used for the heat treatment is provided.Hence, the abrasive coating can be formed again on the rotor blade orthe like, on the site. As a result, even if the abrasive coating isdamaged, repair is easy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart that shows the coating formation method accordingto one embodiment of the present invention;

FIG. 2 is a block diagram for explaining various steps in the coatingformation method in FIG. 1;

FIG. 3 is a perspective view that shows a rotor blade, on which anabrasive coating is formed by the forming method in FIG. 1;

FIG. 4 is an enlarged cross section that shows a part of the rotor bladeshown in FIG. 3; and

FIG. 5 shows a gas turbine having a gas turbine rotor blade, at the tipof which an abrasive coating is formed by the coating formation methodaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An exemplary embodiment of the present invention are explained in detailbelow with reference to the accompanying drawings, however, the presentinvention is by no means limited only to this embodiment. The componentsin the embodiment include one that can be assumed easily by thoseskilled in the art, or substantially the same one.

FIG. 1 is a flowchart of the coating formation method according to oneembodiment of the present invention. This coating formation method isapplied to a case in which a relatively simple apparatus (for example, ahigh vacuum heating furnace) is used, to form an abrasive coating at thetip of the rotor blade of a gas turbine. In this coating formationmethod, at first, a brazing filler metal sheet is prepared (step S1).FIG. 2( a) is an enlarged cross section of a part of this brazing fillermetal sheet, which is comprehensively denoted by reference sign 1. Thisbrazing filler metal sheet 1 comprises a brazing filler metal layer 3 onthe upper side in the figure, an intermediate sticking material layer 5as an adhesive layer, and a released paper 7 on the lower side. Needlessto say, the brazing filler metal layer 3 comprises a brazing fillermetal. If only the sticking material layer 5 and the lower side releasedpaper 7 are provided, the released paper 7 is peeled off, and thebrazing filler metal sheet 1 has only to be adhered on the object to becoated, and hence the work becomes very easy. The thickness of thebrazing filler metal layer 3 is generally from 0.05 mm to 1.00 mm. Thebrazing filler metal layer 3 may be a single sheet or a bundle of two orthree sheets. The intermediate sticking material layer 5, being anadhesive layer, and the lower released paper 7 may be provided accordingto need. When the intermediate sticking material layer 5 and the lowerreleased paper 7 are not provided, the sheet 1 can be adhered on theobject to be coated by pasting or the like, using a binder as a paste.

A preferable brazing filler metal includes one containing boron (B) offrom about 2.75 to 3.50% by mass and composed mainly of nickel (Ni).This brazing filler metal generally contains chromium (Cr) of from about6 to 8% by mass, silicon (Si) of from about 4 to 5% by mass, and iron(Fe) of from about 2.5 to 3.5% by mass. The brazing filler metal sheet 1is preferably one that is not hardened with lapse of time, and aspecific example of the brazing filler metal sheet 1 includes BNi-2 (JISStandard), or the like.

The brazing filler metal sheet 1 is available in the market in the formsuch that the sticking material layer 5 and the released paper 7 arelaminated on the brazing filler metal layer 3 in advance, and thebrazing filler metal contains 83% by mass of nickel, 7% by mass ofchromium, 3% by mass of boron, 4% by mass of silicon, and 3% by mass ofiron.

Subsequently, a coating material layer 9 shown in FIG. 2( b) is formedon the brazing filler metal sheet 1 (step S2). It is also possible toform the coating material layer 9 and the sheet 1 separately, cut themfrom where they were formed, and later stick them together by a binderor the like. In order to prevent cracking such as cut in multiple steps,these are formed as a bilayer with the soft BNi-2, to thereby improvethe sectility of the whole sheet.

The coating material layer 9 is formed by coating a mixture of coatingmaterial particles and a binder 11 on the surface of the brazing fillermetal layer 3. At first, the mixture of the coating material particlesand the binder 11 is poured on the brazing filler metal layer 3.Excessive mixture is scraped off, while spreading out the mixture in theform of sheet by a blade or the like, and coated in a predeterminedthickness, taking the shrinkage allowance into consideration, when themixture is dried. The coating material 9 is dried after coating (stepS3), and generally air-dried for about one day. Because of the drying,the binder 11 volatilizes to some extent, whereby the thickness of thecoating material layer 9 decreases.

The predetermined thickness of the coating material layer 9 aftercoating may be about from 0.10 to 1.00 mm, so that the thickness of thecoating material layer 9 after drying becomes not larger than thecoating thickness of the brazing filler metal sheet 1, as a standard.Therefore, it is preferred to appropriately change the thickness of thecoating material layer 9 after coating the mixture, depending on themixing ratio of the binder 11 and the component ratio of the coatingmaterial layer 9.

MCrAlY particles 13, being a metal material having an oxidationresistance and an intergranular corrosion resistance, are used ascoating material particles, and cubic boron nitride particles 15 areused as the abrasive particles. Hereinafter, it is assumed that when itis simply referred to as coating material particles, this indicates bothof these particles. The binder is mixed with the coating materialparticles and the abrasive particles, to become the coating formationcoating material that forms the coating material layer 9.

MCrAlY is an alloy mainly composed of iron (Fe), nickel (Ni) or cobalt(Co), a chromium (Cr), aluminum (Al), and yttrium (Y), having theoxidation resistance and the intergranular corrosion resistance. It ispreferred to increase the content of Cr and Al, to improve theintergranular corrosion resistance and the oxidation resistance, takingit into consideration that MCrAlY is diluted by the Ni brazing fillermetal, after forming a coating. However, if the quantity of these,particularly, the quantity of Al is too much, the brazing propertydeteriorates, and hence precautions should be taken. In order to improvethe oxidation resistance, the intergranular corrosion resistance, andthe brazing property, Ta, Re, Hf, Si or the like can be added, inaddition to Cr and Al.

It is necessary to bring impurities with respect to the brazingproperty, such as O and N, close to zero infinitely on the surface ofthe MCrAlY particles. As the MCrAlY, it is preferred to use one havingthe particle size in the range of from 10 to 100 μm at random, in orderto increase the filling rate. However, if the particle size is toosmall, the surface area becomes too large, thereby disadvantageouslyincreasing the amount of impurities such as O and N.

On the other hand, as the cubic boron nitride particles 15, one put onthe market by General Electric company, De Beers industrial Diamonds,Showa Denko K. K., Sumitomo Electric Industries, Ltd. and the like canbe used. The cubic boron nitride is classified in single crystal andpolycrystal, and high purity products exist. It is possible to use theright one in the right place, but it becomes clear that one obtained bycoating cubic boron nitride with TiN or the like has excellent brazingproperty. Coating on the cubic boron nitride improves the wettabilitybetween the cubic boron nitride and the brazing filler metal, and hencethe cubic boron nitride particles 15 can be sufficiently buried in thebrazing filler metal. Thereby, dropout of the cubic boron nitrideparticles 15 can be suppressed. As a result, the TBC layer or the likeon the shroud can be shaved off stably, thereby preventing weldingbetween the tip of the rotor blade and the shroud, and enabling highlyreliable operation.

Further, one obtained by coating the cubic boron nitride with Co or Ni,or one obtained by coating the cubic boron nitride with TiN or a Ticompound may be used. It is desired to appropriately select thesedepending on the kinds of the MCrAlY particles forming the coatingmaterial layer 9.

For example, Al₂O₃, SiC, and the like may be used as the abrasiveparticles, instead of the cubic boron nitride particles 15. When Al₂O₃,SiC, or the like is used in order to improve the wettability with thebrazing filler metal, it is preferred to use Al₂O₃ or SiC applied withcoating. From a standpoint of improving the membrane-making with respectto Al₂O₃ and TiN, and the wettability with respect to the MCrAlYmaterial, when Al₂O₃ is used as the abrasive particles, Co, Cr, Ni andthe like can be mentioned as these coating materials. When SiC is usedas the abrasive particles, AlN, TiN, Al₂O₃ and the like can be mentionedas the coating material used for suppressing the reaction with SiC andCr during brazing.

The volume ratio of the volume V_(M) of the MCrAlY particles 13 to thevolume V_(C) of the cubic boron nitride particles 15, V_(M):V_(C), ispreferably from 30:70 to 70:30 inclusive. If the ratio of the cubicboron nitride particles 15 is large, percentage of voids in the brazingfiller metal sheet 1 increases, and the amount of the binder alsoincreases. As a result, insufficient brazing filler metal anddeformation likely occur. If the percentage of the cubic boron nitrideparticles 15 exceeds 70%, that is, the volume ratio V_(M):V_(C) issmaller than 30:70, the density of the cubic boron nitride particles 15is too large, and electric discharge machining from above the coatingbecomes difficult. Since the oxidation resistance also decreases, thereis the possibility that the holding power for the cubic boron nitrideparticles 15 decreases to cause dropout of the cubic boron nitrideparticles 15. From this point of view, it is preferred to set the volumeratio V_(M):V_(C) to not smaller than 1:2, and it is particularlypreferable that the percentage of the cubic boron nitride particles 15be not larger than 60%, that is, the volume ratio V_(M):V_(C) be notsmaller than 40:60, from a standpoint of cost. On the other hand, if thevolume ratio V_(M):V_(C) exceeds 70:30, the polishing ability of theabrasive coating may be insufficient. From this point of view, it ismore preferable to have the volume ratio V_(M):V_(C) of not larger than2:1, and particularly preferable to have the volume ratio V_(M):V_(C) ofnot larger than 60:40. Therefore, the most preferable range of thevolume ratio V_(M):V_(C) is from 40:60 to 60:40 inclusive. At the timeof actual operation, since the specific gravities (densities) of therespective materials are known, the sheet 1 is prepared under masscontrol.

It is also known that the cubic boron nitride particle 15 has highhardness at high temperatures and excellent machinability, butdisappears in a short period of time in a high-temperature oxidativeatmosphere. Therefore, it is necessary to use it by mixing it with SiC,Al₂O₃, and the like having excellent long term stability. These volumeratios are also applicable, when Al₂O₃, TiN and the like are used,instead of the cubic boron nitride particles 15, or together with thecubic boron nitride particles 15.

Various kinds of binders can be used as the binder 11, but it isparticularly preferable to use one that volatilizes at a lowtemperature. The volatile binder 11 volatilizes in the dry step and themelting step described later in detail, and hence hardly remains in theabrasive coating. Therefore, it does not adversely affect the quality ofthe abrasive coating. Further, after the volatile binder 11 volatilizes,a gap is formed. In the melting step described later, since the brazingfiller metal is absorbed in this gap due to the capillary phenomenon,liquid dripping can be considerably reduced. As a result, degradation ofthe rotor blade due to the liquid dripping is suppressed, and atreatment for the liquid dripping (mainly application of stop-off) ishardly required, and hence time and energy for application can beimproved.

Organic binders can be preferably used as the preferable volatile binder11, and particularly, a cellulose binder is more preferable because ofhaving excellent flowability of the brazing filler metal. When a binderin which a plasticizer is added to the binder is used, flexibility isadded to the abrasive coating formation sheet 1 a described later,thereby enabling improvement in the workability, such that the sheetbecomes easy to cut, or the like, which is preferable.

The mass ratio m_(B):m_(C), of the mass ratio m_(B) of the volatilebinder 11 to the mass ratio m_(C) of the coating material particles 13and 15 is preferably from 15:85 to 2:1 inclusive. If the mass ratiom_(B):m_(C) is less than the lowest limit of the above range, there isthe possibility that the coating of the mixture of the coating materialparticles 13 and 15 and the binder 11 becomes difficult. From this pointof view, it is more preferable that the mass ratio m_(B):m_(C) be notsmaller than 20:80 (1:4), and most preferable that the mass ratiom_(B):m_(C) be not smaller than 1:2. On the other hand, if the massratio m_(B):m_(C) exceeds the upper limit of the above range, liquiddripping likely occurs at the time of heat treatment of the object to becoated. From this point of view, it is more preferable that the massratio m_(B):m_(C) be not larger than 60:40, and most preferably, notlarger than 40:60. Therefore, the preferable range of the mass ratiom_(B):m_(C) of the volatile binder 11 to the coating material particles13 and 15 is from 20:80 to 40:60 inclusive.

The coating parameter between the brazing filler metal and the coatingmaterial is preferably from 30:70 to 70:30 inclusive. If this coatingparameter is less than the lowest limit of the above range, the coatingmaterial does not infiltrate in the brazing filler metal at the meltingstep, thereby easily causing insufficient brazing filler metal. Fromthis point of view, it is particularly preferable that the coatingparameter be not smaller than 60:40. After the brazing filler metalsheet 1 has been formed, the sheet 1 is dried at a normal temperature,in order to facilitate the cutting operation, and volatilize theexcessive binder. It is desired to dry the sheet for the entire day orlonger in a thermostatic chamber, if possible, in which the temperatureand humidity are controlled.

The brazing filler metal sheet 1 on which the coating material layer 9is laminated (hereinafter referred to as an abrasive coating formationsheet 1 a) is cut into a predetermined shape and size (step S4). Thecutting means is not particularly limited, but since the abrasivecoating formation sheet 1 a has high brittleness, it is preferred to usea stencil and a ultrasonic cutter. The released paper 7 is peeled offfrom the abrasive coating formation sheet 1 a cut into the predeterminedshape and size, and adhered on the tip of the rotor blade, being theobject to be coated (step S5).

Since the abrasive coating formation sheet 1 a is cut into a bladeshape, the majority thereof remains as unused waste pieces. Sinceexpensive cubic boron nitride is contained in the abrasive coatingformation sheet 1 a, it is necessary to recover the cubic boron nitride.However, a coating for improving the brazing property is applied on thesurface of the cubic boron nitride, and hence it is important to recoverthe cubic boron nitride without damaging this coating. Therefore, inrecovering the cubic boron nitride, it is possible to recover only thecubic boron nitride by boiling and soaking the waste pieces of theabrasive coating formation sheet 1 a in an NaOH solution having aconcentration of about 10% for about 1 to 5 hours to dissolve thebinder, subjecting it to ultrasonic cleaning in pure water, and thenfiltering, washing with pure water, classifying, and drying. Here,ultrasonic cleaning in pure water is performed for about 10 to 30minutes, for example three times, or drying is performed for about onehour, at for example 120° C.

Prior to attaching the abrasive coating formation sheet 1 a, it ispreferred to apply pretreatment, such as blasting, or cleaning by asolvent such as trichloroethylene, acetone or the like, to the tip ofthe rotor blade. It is because the tip of the rotor blade, on which thecoating is formed, becomes rough due to the pretreatment, and oils andfats in the coating formed portion is removed, and as a result,adherence between the coating and the tip of the rotor blade becomesexcellent.

A hole for the cooling medium such as cooling air and cooling steam togush out from the internal cooling passage may be provided at the tip ofthe rotor blade. Therefore, if this hole is closed when the abrasivecoating formation sheet 1 a is attached to the tip of the rotor blade,the cooling medium cannot gush out during the operation of the gasturbine, and hence cooling of the rotor blade may be insufficient.Therefore, the abrasive coating formation sheet 1 a is attached,avoiding the portion of the hole, from which the cooling medium gushesout. However, if the diameter of the hole is small and there are manyholes, it is difficult to avoid these holes. Further, prior to the heattreatment, the abrasive coating formation sheet 1 a is easily cut, andhence it is difficult to ensure the hole in this sheet before attachingthe sheet. Therefore, punching is possible after attaching the sheet, byelectric discharge machining or the like. Punching by the electricdischarge machining is possible, regardless of before or after the cubicboron nitride particles are exposed.

The abrasive coating formation sheet 1 a is then heated together withthe rotor blade body (step S6). A vacuum heating furnace is normallyused for heating. The heating conditions are determined, taking intoconsideration the material of the rotor blade body and the kind of thebrazing filler metal. For example, when the material of the rotor bladebody is a base metal of the rotor blade (Ni group super alloy or thelike), and the BNi-2 is used as the sheet 1 used for the abrasivecoating formation sheet 1 a, at first, the temperature of the vacuumheating furnace is raised from a room temperature to about 600° C. over10 hours or more. Since the abrasive coating formation sheet 1 a isheated, taking long time, to positively volatilize the binder 11 in theabrasive coating formation sheet 1 a at a low temperature, the componentin the binder 11 that is likely to expand by heat does not remain at ahigh temperature. As a result, lines due to thermal expansion do notoccur, and hence the quality of the formed abrasive coating can beimproved. It is desired that the degree of vacuum at this time be higherthan 10⁻⁵ torr. Subsequently, the temperature is raised up to 1000° C.or higher for about 2 hours, and the abrasive coating formation sheet 1a is held in this state for a required period of time. As a result, notonly almost all the binder 11 volatilizes from the coating materiallayer 9, but also a gap is created in the coating material layer 9 afterthe binder 11 has volatilized.

Since the melting point of the brazing filler metal is about 1000° C.,the brazing filler metal melts due to heating at 1000° C. or higher. Thebrazing filler metal that becomes liquid form due to this heating,infiltrates into the gap in the coating material layer 9 due to thecapillary phenomenon, and absorbed in this gap. Boron, being the brazingfiller metal component, also diffuses in the MCrAlY particles 13 in thecoating material layer 9. Since boron lowers the solidifying point ofMCrAlY, MCrAlY becomes a half melted state, and easy to diffuse in thesurrounding brazing filler metal.

Subsequently, the inside of the vacuum heating furnace is cooled to 500°C. or lower by introducing an argon gas or a nitrogen gas (step S7).Thereby, the strength required for the Ni alloy, being the base metal,can be obtained, and as shown in FIG. 2( c), a solidified layer 21 isformed, in which the cubic boron nitride particles 15 are dispersed inthe MCrAlY matrix 19. Since boron disappears to some extent by holdingthe sheet at a controlled temperature of 1000° C. or higher, the meltingpoint of the matrix 19 rises to a temperature at which there is nopractical problem. By this heat history, the heat treatment (stabilizingtreatment) required for increasing the strength of the rotor blade isexecuted. In other words, melting of the coating material and the heattreatment of the rotor blade are completed at the same time at themelting step, by selecting a brazing filler metal having a melting pointlower than the heat treatment temperature of the rotor blade.

Generally, cubic boron nitride has a specific gravity lighter than thatof the brazing filler metal, if the both materials are mixed beforehand,the cubic boron nitride particles 15 float in the surface layer in theliquid brazing filler metal, thereby causing unequal dispersion of thecubic boron nitride particles 15 in the solidified layer 21. Further,liquid dripping of the melted brazing filler metal easily occurs. In thepreferred embodiment of the present invention, the brazing filler metallayer 3 and the coating material layer 9 are sequentially laminated onthe tip 17 of the rotor blade, to mix these by the capillary phenomenon.Therefore, since being held by MCrAlY in the coating material layer 9,the cubic boron nitride particles 15 does not float up, therebydispersion of the cubic boron nitride particles 15 becomes uniform, andliquid dripping of the brazing filler metal can be suppressed.

Subsequently, blasting is applied to the solidified layer 21 (step S8).In the blasting, abrasive blasting particles are sprayed onto thesurface of the matrix 19. By this blasting, as shown in FIG. 2( d), theportion towards the surface of the matrix 19 is removed. Since the cubicboron nitride particles 15 are hardly removed by the blasting accordingto the present invention, the cubic boron nitride particles 15 protrudefrom the matrix 19 (a so-called “exposed”). In this manner, the abrasivecoating 23 is completed. In FIG. 2( d), the boundary between theabrasive coating 23 and the tip 17 of the rotor blade is clearly drawn,but in the actual rotor blade, the boundary between these becomesambiguous due to the dispersion at the time of heating.

In order to remove the portion towards the surface of the matrix 19 byblasting prior to the cubic boron nitride particles 15, it is preferredto use the abrasive blasting particles having hardness lower than thatof the cubic boron nitride particles 15 but higher than that of thematrix 19. In other words, if it is assumed that the Vickers hardness ofthe matrix 19 is H1, the Vickers hardness of the cubic boron nitrideparticles 15 is H2, and the Vickers hardness of the abrasive blastingparticles used for the blasting is H3, it is preferred that H1, H2, andH3 satisfy the relation expressed by the following expression (I):H1<H3<H2  (1).When MCrAlY is used for the cubic boron nitride particles 15 and thematrix 19, for example, Al₂O₃ particles can be used as the abrasiveblasting particles.

If the diameter of the abrasive blasting particles is too large, theexposure of the cubic boron nitride particles 15 becomes insufficient.On the other hand, if the diameter of the abrasive blasting particles istoo small, exposure at the base where the cubic boron nitride particles15 are held becomes too much, thereby the cubic boron nitride particles15 drop out from the coating material layer 9. Therefore, it ispreferred to use abrasive blasting particles having a size smaller thanthe space between the cubic boron nitride particles 15, and a size suchthat the abrasive blasting particles does not attack the base where thecubic boron nitride particles 15 are held. In this example, microblasting using Al₂O₃ particles having a mean particle size of 50 μm isused, but it is desired to appropriately select the diameter of theabrasive blasting particles to be used, based on the particle size ofthe cubic boron nitride particles 15 and spaces therebetween. Forexample, when the space between the cubic boron nitride particles 15 islarge and the surface is rough, it is preferred to use a larger abrasiveblasting particle. Further, in the abrasive coating, when Al₂O₃ and SiCare used in the abrasive particles, it is preferred to use ZrO₂, glassbeads, and the like as the abrasive blasting particle.

FIG. 3 is a perspective view of a rotor blade 25, on which an abrasivecoating 23 is formed by the forming method in FIG. 1. The rotor blade 25comprises a body 27 and a protruding portion 29 extending from the endof the body 27, and the abrasive coating 23 is coated on the upper faceof the protruding portion 29, being the tip of the rotor blade. Thoughnot shown, the internal peripheral face of the shroud is located, facingthe abrasive coating 23, in the gas turbine. When the rotor blade 25 andthe shroud slide with each other, the internal peripheral face of theshroud is polished by the abrasive coating 23. A rotor blade having noprotruding portion 29 may exist, but in this case, the abrasive coatingcan be formed at the tip of the rotor blade.

The mean particle size of the cubic boron nitride particles 15 ispreferably from about 50 to 200 μm. If the mean particle size is lessthan 50 μm, the polishing ability of the abrasive coating 23 may beinsufficient. From this point of view, it is particularly preferablethat the mean particle size be not smaller than 80 μm. On the otherhand, if the mean particle size exceeds 200 μm, not only the coatingthickness of the abrasive coating 23 becomes too large, but also theoxidation resistance of the abrasive coating 23 becomes insufficient.From this point of view, in the case of the rotor blade of the gasturbine, it is particularly preferable that the mean particle size benot larger than 170 μm. Therefore, the most preferable range of the meanparticle size is from 80 to 170 μm.

FIG. 4 is an enlarged cross section of a part of the rotor blade 25shown in FIG. 3. As described above, the cubic boron nitride particles15 protrude from the matrix 19. In this figure, what is shown by twoarrows p is a protrusion size of the cubic boron nitride particle 15.When it is assumed that the mean particle size of the cubic boronnitride particles 15 is D, and a mean value of the protrusion size p inall cubic boron nitride particles 15 protruding from the matrix 19 (thatis, mean protrusion size) is P, the ratio of the mean protrusion size Pto the mean particle size D is preferably from 25% to 70% inclusive. Ifthis ratio is less than 25%, the polishing ability of the abrasivecoating 23 may be insufficient. From this point of view, it is morepreferable that the ratio be not smaller than 30%. On the contrary, ifthis ratio exceeds 70%, the cubic boron nitride particles 15 may oftendrop out from the matrix 19. From this point of view, it is morepreferable that the ratio be not larger than 60%. Therefore, the mostpreferable range of the ratio is from 30 to 60%.

It is preferred that the thickness of the matrix 19 (the portionindicated by the duplex arrow T in FIG. 4) be not smaller than 50 μm. Ifthe thickness of the matrix 19 is less than 50 μm, not only holding ofthe cubic boron nitride particles 15 in the abrasive coating 23 becomesinsufficient, but also the distribution of the cubic boron nitrideparticles 15 cannot be arranged in a form of shark teeth, and hence thelong-time high-temperature durability of the abrasive coating 23decreases.

In this invention, the abrasive coating formation sheet 1 a, being abrazing filler metal sheet 1 laminated with the coating material layer9, is used. This is cut into a predetermined shape and attached to theobject to be coated, and then heat treatment is executed with respect tothe object to be coated, to thereby form the abrasive coating 23 (seeFIG. 3) on the object to be coated. Subsequently, the abrasive coating23 is subjected to the blasting, to expose the cubic boron nitrideparticles 15, so that the abrasive particles protrude from the abrasivecoating 23. After being manufactured in this manner, the abrasivecoating formation sheet 1 a is cut into a predetermined shape andattached to the object to be coated. Thereafter, the abrasive coatingcan be formed only by applying the necessary heat treatment to theobject to be coated. Since blasting is used for exposing the abrasiveparticles, the abrasive particles can be allowed to protrude easily. Asa result, the abrasive coating can be formed very easily, as comparedwith the conventional plating or thermal spraying method.

For example, when the abrasive coating is to be formed at the tip of therotor blade of a gas turbine, according to the coating formation methodof the present invention, the application cost can be suppressed to from1:3 to 1:4 as that of the conventional plating method. Further, the timerequired for the application can be shortened to less than 1:3. Sinceconsiderable effect of decreasing the application cost and reducing theapplication period can be obtained, it is very useful when the abrasivecoating is formed on a large number of rotor blades. Further, sincelarge-scale equipment as in the plating method is not necessary, thecost required for the investment in plant and equipment can be reduced.Since wastewater from plating is not generated as in the plating method,environmental burden can be considerably reduced.

Further, if heating equipment such as the vacuum heating furnace isprepared, the abrasive coating can be formed only by supplying theabrasive coating formation sheet 1 a. Therefore, the heat treatment andthe sheet preparation are not necessarily carried out at the same place.Hence, the freedom in application can be increased. For example, even ina gas turbine plant installed in a location where the applicationfacility does not exist in the vicinity thereof, if only the heatingequipment is provided and the abrasive coating formation sheet 1 a isregularly supplied, recoating and the like can be performed on the site.

In the explanation, an example in which cubic boron nitride and MCrAlYare used as the coating material is shown, but only MCrAlY may be usedas the coating material. In this case, the obtained coating is anoxidation resistant coating. This oxidation resistant coating issuitable for the rotor blade, the stator blade, or the shroud of a gasturbine.

FIG. 5 shows a gas turbine having a gas turbine rotor blade, at the tipof which an abrasive coating is formed by the coating formation methodaccording to the present invention. The air taken in from an air intake50 is compressed by a compressor 51 to become high temperature and highpressure compressed air, and is fed to a combustor 52. The combustor 52supplies a gas fuel such as natural gas or the like, or a liquid fuelsuch as gas oil, light fuel oil or the like to the compressed air, toburn the fuel, to thereby generate high temperature and high pressurecombustion gas. This high temperature and high pressure combustion gasis guided to a combustor tail pipe 53, and injected to a turbine 54.

The turbine 54 comprises a rotor blade 25 (see FIG. 3), at the tip ofwhich an abrasive coating is formed by the coating formation methodaccording to the present invention. The rotor blade 25 has a coatingaccording to the present invention formed at the tip thereof. When theoperation of a gas turbine 100 is started, a so-called initial slidingoccurs due to thermal expansion of the rotor blade, and the tip of therotor blade 25 may come in contact with the internal wall of a shroud55. When a certain period of time has passed since starting theoperation, the tip of the rotor blade 25 may come in contact with theinternal wall of the shroud 55 due to the deformation of the shroud 55,to thereby cause a so-called secondary sliding. In either case, sincethe abrasive particles are firmly brazed at the tip of the rotor blade25, by the coating formation method of the present invention, thecoating of TBC or the like (not shown) formed on the internal wall ofthe shroud 55 can be shaved off. As a result, welding of the rotor blade25 can be prevented, and the gas turbine 100 can be stably operated. Itis preferred that cubic boron nitride is allowed to function withrespect to the initial sliding, and SiC and Al₂O₃ having excellentlong-term stability at high temperatures are allowed to function withrespect to the secondary sliding. Therefore, it is more desirable to mixand use these, for ensuring the long-term reliability of the gasturbine.

The rotor blade 25 according to the present invention has a coatingformed at the tip thereof by brazing, and hence abrasive particles suchas cubic boron nitride and the like can be distributed in a form ofshark teeth. Therefore, even if the abrasive particles on the surfacedrop out, the next abrasive particles will appear. Hence, the coating ofTBC or the like formed on the internal wall of the shroud 55 can beshaved off stably, until the abrasive coating 23 (see FIG. 3)disappears. As a result, more reliable operation can be realized thanthe case of using a rotor blade, on which a coating is formed by theconventional plating or thermal spraying method.

The coating formation method of the present invention includes:

-   (1) a lamination step of laminating a brazing filler metal layer    composed mainly of a brazing filler metal and a coating material    layer composed mainly of a coating material, on the surface or the    back of the object to be coated;-   (2) a melting step of heating the laminated brazing filler metal    layer and the coating material layer to diffuse the coating material    and the brazing filler metal, while allowing the brazing filler    metal component to melt and infiltrate in the coating material; and-   (3) a fixing step of solidifying the molten brazing filler metal to    fix it on the object to be coated, and a coating is formed by a    so-called brazing. This method can be executed at a low cost, as    compared with the plating or thermal spraying method, and does not    require large-scale equipment. As a result, there is the effect that    there is little limitation on the application site.

In the coating formation method according to the present invention, thecoating parameter between the brazing filler metal and the coatingmaterial laminated at the lamination step is set to from 30:70 to 70:30inclusive. Therefore, not only the brazing filler metal is reliablymelted at the melting step, but also the formed coating is firm, therebyimproving the property thereof.

In the coating formation method according to the present invention,since the brazing filler metal contains boron, boron diffuses in thecoating material at the melting step, to allow the solidifying point ofthe coating material to fall. Therefore, even when the coating materialis heated at a relatively low temperature, the coating material can bereliably melted, at a low cost.

In the coating formation method according to the present invention, abrazing filler metal having a melting point lower than the heattreatment temperature of the object to be coated is selected. As aresult, the melting step can be executed at the same time with the heattreatment of the object to be coated. Hence, the work efficiency informing the coating can be improved.

In the coating formation method according to the present invention,since the coating material layer, in which coating material particlesare dispersed in the binder, is used, lamination of the coating materialbecomes easy by the binder. The binder substantially completelyvolatilizes at the melting step, and hence degradation of the coatingresulting from the remaining binder in the coating can be suppressed.

In the coating formation method according to the present invention, themass ratio between the binder and the coating material particles is from15:85 to 2:1 inclusive. As a result, formation of the coating materiallayer becomes easy, and liquid dripping of the brazing filler metal atthe melting step can be suppressed, thereby enabling improvement of theworkability.

In the coating formation method according to the present invention,MCrAlY particles and cubic boron nitride particles are used as the maincomponent. In the abrasive coating obtained by this coating materiallayer, cubic boron nitride serves as abrasive particles, and MCrAlYbecomes a matrix to fix the abrasive particles. The MCrAlY matrix cansuppress oxidation of the abrasive particles.

In the coating formation method according to the present invention, thevolume ratio between the MCrAlY particles and the cubic boron nitrideparticles is from 1:2 to 2:1 inclusive. Therefore, the polishing abilityof the abrasive coating is improved and the abrasive particles can bereliably fixed.

In the coating formation method according to the present invention,since the abrasive coating is formed at the tip of the rotor blade of agas turbine, cubic boron nitride in the abrasive coating polishes theinternal peripheral face of the opposite shroud, and hence a damage ofthe rotor blade can be prevented.

In the coating formation method according to the present invention, anexposure step of removing a part of MCrAlY from the surface of the fixedcoating material layer to expose the cubic boron nitride particles isincluded. Further, in the coating formation method according to thepresent invention, blasting is used at the exposure step of exposing thecubic boron nitride particles. As a result, exposure of the cubic boronnitride particles can be appropriately performed.

In the coating formation method according to the present invention, inthe blasting, an abrasive harder than the MCrAlY particles but softerthan the abrasive particles is used. As a result, since MCrAlY can beremoved efficiently from the formed abrasive coating, the abrasiveparticles can be exposed sufficiently.

In the coating formation method according to the present invention, inthe blasting, an abrasive having a particle size smaller than theparticle size of the abrasive particles and smaller than the spacesbetween the abrasive particles. As a result, since dropout of theabrasive particles can be suppressed to a minimum, while sufficientlyexposing the abrasive particles, sufficient polishing performance can beexhibited from the initial stage.

In the coating formation method according to the present invention,other examples of preferable coating material layers include onecomposed mainly of the MCrAlY particles. An oxidation resistant coatingobtained by this coating material layer can be preferably used invarious members of a gas turbine where high-temperature gas circulates,more specifically, in a rotor blade, a stator blade, and a shroud, as inthe coating formation method according to the next invention.

In the coating formation method according to the present invention,since abrasive particles, a metal material having at least an oxidationresistance, and a binder are contained, the brazing filler metal isabsorbed in the gap produced by volatilization of the binder, in theheat treatment at the time of coating formation. Thereby, liquiddripping of the brazing filler metal can be considerably reduced, andhence the quality after forming the coating on the object to be coatedcan be improved. Further, since the metal material has the oxidationresistance, oxidation hardly occurs even in a high temperaturecombustion gas atmosphere in which the rotor blade of the gas turbine isused. As a result, the abrasive particles can be reliably held, andstable polishing performance can be exhibited even in long-term use, anda reduced thickness due to oxidation of the base metal can be prevented.Hence, more stable operation of the gas turbine can be realized.

In the coating formation coating material according to the presentinvention, in the coating formation coating material, a ratio betweenthe mass of the binder and the mass of the abrasive particles and themetal material is from 15:85 to 2:1 inclusive. As a result, the coatingmaterial layer can be formed easily, and liquid dripping at the meltingstep can be suppressed.

In the coating formation coating material according to the presentinvention, the metal material contained in the coating formation coatingmaterial is MCrAlY. Since MCrAlY having an oxidation resistance and anintergranular corrosion resistance is used, even when a coating isformed on the rotor blade of a gas turbine used in a high-temperatureoxidative atmosphere, the abrasive particles can be held for long timeto maintain the polishing performance. As a result, stable operation ofthe gas turbine can be realized.

In the coating formation coating material according to the presentinvention, in the coating formation coating material, the volume ratiobetween the MCrAlY particles and the abrasive particles is from 1:2 to2:1 inclusive. Therefore, the occurrence of insufficient brazing fillermetal can be prevented, and the workability can be improved. Further,since abrasive particles can be sufficiently fixed, dropout of theabrasive particles can be suppressed, thereby enabling reliableoperation of the gas turbine.

In the abrasive coating formation sheet according to the presentinvention, a brazing filler metal and any one of the coating formationcoating materials described above are laminated. Therefore, after thisabrasive coating formation sheet is attached to the object to be coated,the abrasive coating can be formed only by heat-treating the object tobe coated. Hence, the abrasive coating can be formed considerablyeasily, as compared with the plating or thermal spraying method.Further, since a treatment prior to the heat treatment is completed onlyby attaching the abrasive coating formation sheet to the object to becoated, the operation becomes very easy. Further, since it is a sheetform, it can be appropriately cut according to the shape of the objectto be coated. Therefore, it can easily correspond to objects to becoated having various shapes.

In the abrasive coating formation sheet according to the presentinvention, in the abrasive coating formation sheet, the coatingparameter between the brazing filler metal and the coating formationcoating material is set to from 30:70 to 70:30 inclusive. Therefore, notonly the coating formation coating material reliably melts at themelting step, but also the formed coating becomes firm.

In the abrasive coating formation sheet according to the presentinvention, in the abrasive coating formation sheet, boron is containedin the brazing filler metal. Since boron is contained, this borondiffuses in the coating formation coating material at the melting step,to allow the solidifying point of the coating formation coating materialto fall. Therefore, even when the coating formation coating material isheated at a relatively low temperature, the coating formation coatingmaterial melts. After boron diffuses, since the melting point of thecoating formation coating material increases, the heat resistance of thebrazing filler metal increases. As a result, even when the coatingformation coating material is used in a high-temperature gas, such as inthe rotor blade and the shroud of a gas turbine, the brazing fillermetal can be used without melting.

In the abrasive coating formation sheet according to the presentinvention, in the abrasive coating formation sheet, the brazing fillermetal is selected from materials having a melting point lower than theheat treatment temperature of the object to be coated. As a result, themelting step is allowed to progress at the same time with the heattreatment of the object to be coated.

In the abrasive coating formation sheet according to the presentinvention, in the abrasive coating formation sheet, an adhesive layer isformed on the brazing filler metal. Therefore, so long as the abrasivecoating formation sheet is prepared, the treatment prior to the heattreatment is completed only by adhering the abrasive coating formationsheet to the object to be coated, without requiring pasting and waitingfor drying of the paste. As a result, time and energy for coatingformation can be reduced.

In the rotor blade of a gas turbine according to the present invention,an abrasive coating is formed at the tip thereof by any one of thecoating formation methods. Therefore, the abrasive coating can be formedvery easily, as compared with the plating or thermal spraying method. Asa result, the time required for coating formation can be considerablyreduced, as compared with the coating formation method described above,and the production cost thereof can be reduced.

In the rotor blade of a gas turbine according to the present invention,any one of the abrasive coating formation sheets is adhered to the tipthereof. Therefore, the abrasive coating can be formed only byperforming the necessary heat treatment on the rotor blade, and hencethe abrasive coating can be formed considerably easily, as compared withthe plating or thermal spraying method. As a result, the time requiredfor coating formation can be considerably reduced, as compared with thecoating formation method described above, and the production costthereof can be reduced.

In the gas turbine according to the present invention, a turbine drivenby a combustion gas injected from the combustor comprises the rotorblade. Therefore, if only heat treatment equipment is provided, anabrasive coating can be easily formed on the rotor blade. Hence, evenwhen there is no plating equipment near the operation site of the gasturbine, the abrasive coating can be easily formed, so long as a heatingfurnace used for heat treatment is equipped. As a result, since theabrasive coating can be formed again on the rotor blade on the site,repair of the rotor blade is easy.

INDUSTRIAL APPLICABILITY

The coating formation method, the coating formation material, theabrasive coating formation sheet, the rotor blade of the gas turbine, onwhich an abrasive coating or the like is formed by the coating formationmethod, and the gas turbine using this rotor blade according to thepresent invention are useful in forming an abrasive coating, anoxidation resistant coating or the like, which is formed on a membersuch as a rotor blade, a stator blade, or a shroud in a combustionengine (gas turbine, jet engine, and the like) and a steam turbine, andis suitable for easily forming these coatings.

1. A coating formation method comprising: a lamination step oflaminating a brazing filler metal layer and a coating material layer ona surface of an object to be coated, the brazing filler metal layerbeing composed essentially of a brazing filler metal, and the coatingmaterial layer comprising a coating material having abrasive particles;a melting step of heating the laminated brazing filler metal layer andthe coating material layer to allow at least a part of the coatingmaterial to melt, while diffusing the brazing filler metal in thecoating material; and a fixing step of solidifying the melted coatingmaterial to fix it on the object to be coated.
 2. The coating formationmethod according to claim 1, wherein a coating parameter between thebrazing filler metal and the coating material laminated at thelamination step is from 30:70 to 70:30 inclusive.
 3. The coatingformation method according to claim 1, wherein the brazing filler metalcomponent diffused in the coating material is boron.
 4. The coatingformation method according to claim 1, wherein the brazing filler metalis selected from materials having a melting point lower than a heattreatment temperature of the object to be coated.
 5. The coatingformation method according to claim 1, wherein the coating materiallayer is one in which coating material particles diffuse in a binder. 6.The coating formation method according to claim 5, wherein a mass ratiobetween the binder and the coating material particles is from 15:85 to2:1 inclusive.
 7. A coating formation method comprising: a laminationstep of laminating a brazing filler metal layer and a coating materiallayer on a surface of an object to be coated, the brazing filler metallayer being composed essentially of a brazing filler metal, and thecoating material layer being composed essentially of a coating material;a melting step of heating the laminated brazing filler metal layer andthe coating material layer to allow at least a part of the coatingmaterial to melt, while diffusing the brazing filler metal in thecoating material; and a fixing step of solidifying the melted coatingmaterial to fix it on the object to be coated, wherein the coatingmaterial layer has MCrAlY particles and abrasive particles, the abrasiveparticles being selected from a group consisting of cubic boron nitride,Al₂O₃, and TiN.
 8. The coating formation method according to claim 7,wherein a volume ratio between the MCrAlY particles and the abrasiveparticles is from 1:2 to 2:1 inclusive.
 9. The coating formation methodaccording to claim 7, wherein the object to be coated is a tip of arotor blade of a gas turbine.
 10. The coating formation method accordingto claim 7, further comprising: an exposure step, which is executedafter the fixing step, of removing a part of MCrAlY from a surface ofthe fixed coating material layer to expose the abrasive particles. 11.The coating formation method according to claim 10, wherein the exposurestep is carried out by blasting.
 12. The coating formation methodaccording to claim 11, wherein in the blasting, an abrasive harder thanthe MCrAlY particles but softer than the abrasive particles is used. 13.The coating formation method according to claim 12, wherein a particlesize of the abrasive is smaller than a particle size of the abrasiveparticles and smaller than a space between the abrasive particles. 14.The coating formation method according to claim 1, wherein the coatingmaterial layer comprises MCrAlY.
 15. The coating formation methodaccording to claim 14, wherein the object to be coated is a rotor blade,a stator blade, or a shroud of a gas turbine.
 16. A method of providinga clearance between a blade and a shroud, comprising: laminating abrazing layer and a coating layer on a surface of one of the blade andthe shroud, the coating layer comprising abrasive particles; heating thelaminated brazing layer and coating layer to allow at least a portion ofthe brazing layer to diffuse into the coating layer; cooling the heatedlaminated brazing layer and coating layer to form an abrasive layer; andmoving the blade relative to the shroud to remove material with theabrasive layer disposed on the one of the blade and the shroud from theother of the blade and the shroud.
 17. The method according to claim 16,wherein the abrasive layer is formed on the blade.
 18. The methodaccording to claim 17, wherein the blade comprises a rotor blade of aturbine.
 19. The method according to claim 18, wherein moving comprisesrotating the rotor blade about an axis.
 20. The method according toclaim 16, wherein the abrasive particles comprise one of cubic boronnitride, Al₂O₃, and TiN.
 21. The method according to claim 16, whereinthe coating layer comprises a coating material.
 22. The method accordingto claim 21, wherein the abrasive particles comprise one of cubic boronnitride, Al₂O₃, and TiN.
 23. The method according to claim 22, whereinthe coating material comprises MCrAlY.
 24. The method according to claim23, further comprising: exposing the abrasive particles on the one ofthe blade and the shroud.
 25. The method according to claim 24, furthercomprising: blasting the abrasive layer to expose the abrasive particleson the one of the blade and the shroud.
 26. A brazed layer, comprising:abrasive particles comprising at least one of cubic boron nitride,Al₂O₃, and TiN; a metal material resistant to oxidization; and a binder,wherein a ratio between a mass of the binder and a mass of the abrasiveparticles and the metal material is from 15:85 to 2:1 inclusive, whereinthe metal comprises MCrAlY.
 27. The brazed layer according to claim 26,wherein a volume ratio between MCrAlY particles and the abrasiveparticles is from 1:2 to 2:1 inclusive.